In order to make a shaped part of sheet metal, for instance for use in a motor vehicle, it is known from German 24 52 486 of C. Ridderstrale to heat the sheet to above the AC3 annealing temperature and then to deform it in less than 5 sec in a machine between a pair of cooled tools. The workpiece is held after deformation between the tools to quench-harden it. This method only works well for steel parts with a strong grain structure and good dimensional stability.
More complexly shaped parts cannot be done in a single deforming step. Thus the workpiece is moved from a preworking machine, a forging press for instance, where it gets an intermediate shape, to a final press where it is given its final shape and hardened. The preworking takes place on a workpiece that is well below annealing temperatures as a sheet-metal workpiece has only limited capacity to hold heat. Thus the preformed workpiece must be reheated before being pressed into its final shape.
Parts used in motor vehicles must have, in addition to a shape of exactly defined dimensions and the necessary hardened grain structure, some form of corrosion protection. Thus EP 1,013,785 of J. Laurent proposes coating a hot-rolled workpiece with an appropriate protective metal or alloy before the preworking step. Subsequent deformation at high temperature as described above produces an intermetallic phase between the protective coating and the underlying steel so as to prevent decarbonization. The hardening process also improves the surface hardness of the protective metal coating. When the protective metal is aluminum or an aluminum alloy, it can easily withstand annealing temperatures. Such a coating, however, becomes brittle and can separate or spall off when worked cold.
It is therefore recommended to reheat such coated workpieces. Thus it is necessary to reheat the workpiece after the preworking step. It is well known (see “Umformtechnik” Springer Verlag 1988, volume 2, chapter 3.4.2.4) to use the heat generated in a forging operation for hardening. This works with relatively massive workpieces capable of holding heat, but not with thin sheet-metal workpieces.